This invention relates to torque-responsive or torque-measuring tools and more particularly to a digital torque-responsive pneumatic tool, and specifically a torque-responsive, automatically torque-controlling digital pneumatic torque tool of portable character.
Pneumatic tools, i.e., air-powered tools, are widely used in automotive repair and maintenance. They also are used in manufacturing and assembly and in many other technical areas, as in marine repair and maintenance as well as also aircraft repair and maintenance.
Portable tools of the above type are often called pneumatic torque tools and also pneumatic torque wrenches. They are used selectively to apply torque (i.e., a twisting moment) to a bolt, screw, stud, nut, or other fastener, or joint, such as shear joints and tensile joints. Any and all of the foregoing elements subjected to torque are in this description referred to for convenience as "workpiece." For example, in automotive repair and maintenance (which may simply be referred to in this description as "automotive use") pneumatic torque wrenches may be used to secure various kinds of fasteners. An example is the use of portable pneumatic torque wrenches used to tighten and untighten lug nuts, which fasten a wheel to a wheel flange, as during installation or replacement of wheels.
In these various kinds of uses, the user typically desires to be able to apply a desired torque to the workpiece, which often can presently only be measured by the use of a calibrated torque wrench. The user specifically needs, and surely would like to be able, to measure the torque actually applied to a workpiece, as the torque is being developed by the tool. Thus, various manual torque tools are used, including click wrenches, dial wrenches, bending beam wrenches, and torque limiting wrenches, which may indicate a predetermined torque by detent disengagement or "break" action. When using pneumatic torque wrenches, which are notable for their speed and convenience as compared to the foregoing manual torque tools, the user most preferably surely would like to control use of such wrenches so that a workpiece, whether a nut, screw or other object, will be tightened only to a predetermined torque, as by automatically controlling the supply of air to the tool.
Present pneumatic torque wrenches sometimes are misused when applying fasteners for vehicle wheels, namely wheel lug nuts or wheel screws, for example. Therefore, it is common to encounter a problem of the lug nuts being too tight to be removed by vehicle users. There can also be a serious problem of stretching of lug nuts or wheel screws from over-tightening, which weaken the lug nuts and present a risk of stress fatigue and failure resulting from exceeding elastic limits of steels and other alloys from which these fastening elements are formed, or structures into or upon which they must be threaded. Among the various problems which can result from applying excessive torque to a workpiece are nut dilation, stud or fastener stretching or creep, embedment, galling, plastic deformation, and excessive torsional or tensile train strain.
To limit torque applied by pneumatic torque wrenches, tools have been equipped with air controls, which, by limiting air pressure provided to an air motor of the tool, can provide some approximate measure of torque control. But, in fact, an arrangement for merely limiting air flow does not provide sufficient precision of control, absent the use of feedback or other torque sensing by which the actual torque being developed by the tool is measured while torque is being applied.
In continuous drive industrial tools in which torque is to be controlled precisely, as during the manufacture of engines, it has been known to provide a system in which a torque reaction is provided either by an operator or a fixture. Such systems have characteristic problems, and also may cause operator fatigue or repetitive stress injury. Discontinuous drive tools have also been proposed which use a pulse mechanism; but they do not automatically control the application of torque. Rather, discontinuous drive tools attempt to limit torque through a pulsing action. They do not offer a wide range of setting of preset torque, and may not prove accurate, and may be difficult to audit.
Another type of industrial tool used to apply torque to a workpiece has used electronic sensors, for example as incorporated in torque transducer accessories which may be interposed between a power tool and workpiece or fixture, and which can sense and electronically measure the degree of torque being applied to the workpiece. In one such sensor arrangement, it has been known to use multiple coils about a rotating torque-applying shaft, which measure torque by sensing magnetostrictive forces developed in the shaft. The shaft is prepared with a ferrite coating by using chemical vapor deposition or plasma spray to deposit ferritic material on regions of the shaft. By combining exciter and sensor coils spaced along the shaft over the ferrite-coated portions of the shaft, differential sensing of magnetostrictive changes produced in the coated regions in response to the development of torque is used to generate a signal which represents the magnitude of torque developed. Circuits responsive to the torque signal are then used to control the torque-producing motive force applied to the shaft, and thereby control the torque delivered to a workpiece by the shaft. Such a system is both complicated and expensive to manufacture; and thus not been practically usable for pneumatic torque wrenches of portable type such as are used by automotive repair and maintenance facilities, tire installers/repairers, and other users who require portability and economy in a pneumatic tool.